Prove that f(x)=kx

f is a Lebesgue measurable function on E,m(E) is finite.f(x+y)=f(x)+f(y). Prove that f(x)=kx.
By Lusin's theorem,I can find a closed set F,f is continuous on F and m(E-F)is very small.I don't know what to do next...